Ah yes, the infamous "tick", the greatest fear of every new '03-04 Cobra owner. Has to do with flawed head casting and coolant flow design on the first revision of the head that causes coolant to dead end in the back of the driver side head and causes the 7 & 8 cylinders to run hotter than the others. The ticking sound occurs when the valve guides and/or seats warp due to running too hot and the ticking itself is the sound of the valves not seating correctly and tapping because of it.
It's only on the DOHC 4v heads for 03-04 and is more prominent on the Cobra and Mach 1 due to their high performance nature and tendency to be driven hard. Either the head can be replaced with a revised casting (usually referred too as "Jan '05" heads) under warranty (or discounted cost if out of warranty) or you can install an after market bypass kit for about $200 that replaces the freeze plug on the driver side head with an elbow and hose and allows the coolant to flow out the back and merge with the exit path that IS on the passenger side head.
I've had two '03 Cobras, the first heavily modified and both driven hard. My coupe is sitting at 40k miles and is completely stock. Build number on the coupe is #50 and it was made in '02 and the earlier ones were the ones most vulnerable to it. But I haven't had any problems on either car. All in all its very rare and nothing to worry about, and most cases of it occurring are after the vehicle has been driven very hard in hot weather, top speed pulls, or modified to make crazy horsepower without any kind of cooling system upgrades, etc. If you are paranoid about it, I'd suggest looking for a more recent build date and not anything with that engine made in 2002 - early 2003.
It's a bad thing to end up with, but the chances are slim. I wouldn't worry about if. If you buy a used car that has 20-40k miles or more on it, and you let it get to full temp and test drive it and pop the hood and don't hear anything then you're fine. (it's very loud you will know it when you hear it, don't confuse with injector tick which is normal). Basically if it hasn't developed by then, it won't at all.